Gasification is a high-temperature process usually conducted at elevated pressure to convert carbon-containing materials into carbon monoxide and hydrogen gas. Since this gas is often used for the synthesis of chemicals or synthetic hydrocarbon fuels, the gas is often referred to as “synthesis gas” or, more succinctly “syngas”. Syngas can be used as a fuel to generate electricity or steam and as a source of hydrogen.
Typical feeds to gasification include petroleum-based materials that are neat or residues of processing materials, such as heavy crude oil, bitumen recovered from tar sands, kerogen from oil shale, coke, and other high-sulfur and/or high metal-containing residues; gases; and various carbonaceous waste materials. Dry or slurried feedstock is reacted in the gasifier in a reducing (oxygen-starved) atmosphere at high temperature and (usually) high pressure. The resulting syngas typically contains about 85 percent of the feed carbon content as carbon monoxide, with the balance being a mixture of carbon dioxide and methane.
Three basic types of gasifiers are: fixed bed, fluidized bed and entrained flow. The fixed bed gasifier is operated at relatively low outlet temperature (425° C.-600° C.) and requires a lesser amount of oxygen compared to the other two types of gasifiers; however, the product syngas contains substantial unconverted methane, and by-product tars and oils. The fluidized bed gasifier operates at more moderate outlet temperatures (900° C.-1050° C.) and requires a greater amount of oxygen than a comparable fixed bed gasifier. While the synthesis gas from a fluidized bed gasifier is of higher purity, the carbon conversion is lower than a comparable entrained flow gasifier which operates at much higher temperatures (1250° C.-1600° C.) and requires significantly higher energy input, but from which synthesis gas of the highest purity can be obtained.
The high temperature in the entrained gasifiers and in the lower zones of certain fixed bed gasifiers converts the inorganic materials in the feed into a molten vitrified material which solidifies when removed from the gasifier, producing a material resembling coarse sand and generally referred to as slag. Fluid bed gasifiers produce dry ash which is not vitrified but only consolidated or agglomerated. Depending on the gasifier, it is desirable either to remove ash at lower temperatures (non-slagging gasifiers) or as a low viscosity liquid at high temperatures (slagging gasifiers). This inert slag or ash has a variety of uses in the construction and building industries.
The raw syngas can be treated using proven commercial technologies to remove trace elements and other impurities for recovery or recycle to the gasifier. Sulfur can be recovered as marketable elemental sulfur or sulfuric acid. In addition to a fuel source, syngas can be used as a raw material in the production of fuels, chemicals, fertilizers, and industrial gases.
The present embodiments are detailed below with reference to the listed Figures.